There is no admission that the background art disclosed in this section legally constitutes prior art.
In certain types of beam writing systems, such as systems used for lithography, beams of energy are directed onto a coated substrate for interacting with it to form desired patterns. During a scan sequence of the beam, the pattern exposure is divided into discreet flashes, which are directed onto selected pattern areas of the substrate. The manner in which the dose, exposure sequence and shape of the flash are applied is referred to as a writing strategy.
There have been basically two types of writing strategies; namely, raster scan and vector scan systems. Prior known raster Gaussian beam writing strategies, for example, have been used in connection with particle beams such as electron beams, wherein the beam is scanned in raster manner over a field of view such as on a substrate. The beam is selectively blanked and unblanked as it is being scanned over the field. During the unblanking of the beam, a flash of the beam is directed onto the substrate and is modulated in coordination with the scanning in order to generate a pattern. The flash size is approximately the size of the address unit of the printing grid (set by the beam modulation). The flash has a substantially Gaussian profile in order that adjacent flashes blend smoothly together. The limited flash profile limits feature resolution and critical dimension (CD) uniformity.
A raster Gaussian beam has the advantage of an excellent pattern placement accuracy, and has a readily predictable writing speed. However, improving the resolution requires reducing the basic address unit. Such reduction results in the lengthening of the printing time quadratically.
The raster Gaussian beam method has been improved by permitting overlapping flashes and gray modulation for finer edge placement. However, the fundamental limitations (feature resolution and CD uniformity) have not been alleviated thereby.
A vector shaped beam strategy method is another writing strategy and includes a variable shape scanned in a vector fashion. The flash is modulated in coordination with the vectoring in order to generate a pattern. The flash size is independent of the addressing of the vector. The flash profile for a given flash size is much steeper than that of a correspondingly sized raster Gaussian beam flash. This improves feature resolution and critical dimension uniformity.
A vector shaped beam method may have the disadvantage for some applications of slow flash rate. The slow flash rate may be due to the use of high precision digital to analog converters (DAC) required for vectoring the beam.
A further writing strategy technique is a raster shaped beam method, which is a hybrid technique. In this regard, reference may be made to U.S. Pat. Nos. 5,876,902 and 6,262,429. It employs a raster scanned, shaped beam. As in the case of a vector shaped beam, a flash profile is steep, yielding good resolution and CD uniformity. As is the case for raster Gaussian beam, the scan is rastered, yielding excellent placement accuracy. Furthermore, the flash rate can be much higher than that employed in a vector shaped beam system, because only low resolution DACs are required.
A raster shaped beam system unfortunately may retain the disadvantage for some applications of the raster Gaussian beam system. In this regard, further improvements to the feature resolution may, for certain applications, quadratically lengthen printing time, or at least lengthen it significantly. This is as a result of each flash being confined substantially within a grid cell.
For example, in U.S. Pat. No. 5,876,902, a horizontal line shaped beam flash, having a maximum length of one flash origin field cell, can be exposed from a single raster scan origin only. Thus, the patented technique is designed to overlap flash exposures of the beam in the X coordinate. With such a technique, while being successful for certain applications, with patterns which have adjacent flash locations in the X direction, the patented method would require the adjacent flash location to be exposed during a subsequent scan line. Because of this operation, unwanted high frequency noise due to the deflection of the beam can be introduced for certain patterns.